On the 8TKUCTDRE AND CLASSIFICATION OP THB TrEMATASPIDAB. 25 



its iiervc, and by the origin of tlie nerve in the brain. In Limiilus, the organ was unquest- 

 ionably originally a visual organ serially homologous with the lateral and median eyes. 

 In the adult it lies on the ventral side in front of the chelicerac. It gradually loses the 

 histological characters of a visual organ and finally presents many points of resemblance to 

 the olfactory organ of a Vertebrate. The hypostomeal eyes of Trilobites are very probably 

 homologous with the olfactory organs of Limnlus. In Branchipus, the same organ has moved 

 from its original position on the ventral surface to a point on the dorsal surface almost as 

 far back as the median ocellus. In Apus, the two organs have moved still farther back and 

 have united behind the median ocellus to form an unpaired organ in precisely the same 

 location as the post-orbital opening of Tremataspis. 



G. The Oral Plates have been worked out anew from the single fossil and its cast 

 that was used by Rohon. My description differs from his in several very important respects. 



According to my interpretation of this important fossil, there are on each side nine 

 oral plates arranged in four rows. The marginal plates are provided with one or more 

 rounded incisions lying opposite corresponding incisions of the ventral and dorsal shields. 

 The marginal plate of the fourth row is probably a compound plate. There is a row of four 

 or five small plates on each side, just behind the anterior margin of the dorsal shield. 



A small triangular median plate lies in the anterior oral region. Its anterior margin 

 seems to be articulated to the hinge- like process on the anteiior median margin of the 

 dorsal shield. The outer surface of the plate is smooth and provided with a low keel-like 

 ridge that gives it the appearance of the rostrum of an Arthropod. Its apex lies consider- 

 ably below the surrounding surface in a median depression that probably leads into a small, 

 circular oral cavity like that of an Arachnid. There is no trace of a large transverse oral 

 opening between the anterior plates and the anterior ventral margin of the dorsal shield. 



H. The Anterior Margin of the Dorsal Shield is deflected sharply downwards to form 

 a low wall in front of the oral region. A small keel lies in the median line on the posterior 

 surface of this wall with its rounded edge directed backwards. Two rounded, tooth-like pro- 

 jections of the rim directed ventrally, lie on either side. The posterior lateral margins of 

 the dorsal shield are folded sharply toward the ventral median line and present three large 

 rounded incisions that continue forward the series of six incisions of the ventral shield de- 

 scribed by Schmidt and Rohon. 



/. The Appendages: The most anterior incision is the largest and is clearly the same 

 as that so well seen on the margin of the dorsal shield in Tolijpaspis^ Oyathaspis and I*tcr- 

 aspis, and which has been regarded as an opening for the lateral eye. Lindstrom's im- 

 portant discovery of an appendage in Gyathaspis, my own discovery of the appendages in 

 Trcmataspis, and a renewed examination of the Pteraspids in the British Museum indicate 



3u. •b.-Um. Otx. ^ 



